This invention relates to a combustor for gas turbines.
It is known that NO.sub.x (nitrogen oxides) is produced during combustion by the reaction of oxygen and nitrogen present in the atmosphere, and that this reaction may be reduced by a low-oxygen combustion at low temperatures.
In a co-pending application for Letters Patent of the United States, Ser. No. 828,100, filed Aug. 22, 1977, combustor for gas turbines is described which comprises a cylindrical tube and a front inner tube disposed coaxially within the cylindrical tube and cooperating therewith to define a combustion chamber, a nozzle for injecting the fuel into the combustion chamber, means for creating a swirling stream of the air within the combustion chamber, the front inner tube having a continuously or stepwise increasing diameter from the nozzle towards the downstream side of the combustion chamber, the larger end of the front inner tube being in sliding engagement with the inner surface of the cylindrical tube. The front inner tube and the cylindrical tube are provided with a plurality of air supply openings. 0.8 to 1.2 times the theoretical amount of air (Ao) is fed through primary air supply openings and 1.7 Ao to 2.5 Ao is fed through the secondary air supply openings, while 2.0 Ao to 2.7 Ao is supplied as diluting air to the downstream area of the combustion chamber.
One essential disadvantage of that combustor for gas turbines resides in the fact that due to insufficient cooling of the combustion gas in the portion of the combustion chamber along the central axis thereof, the production of NO.sub.x cannot sufficiently and effectively be reduced. The major portion of the air fed into the combustion chamber through the air supply openings provided in the front inner tube flows along and close to the interior wall of the front inner tube, while only a small portion of the air will reach the central axis of the combustion chamber to cool the combustion gases in that zone. As a result, the central portion of the combustion chamber is left at high temperatures so that large amounts of NO.sub.x are produced.
Another essential disadvantage of the combustor referred to above is the fact that the amount of air supplied through the openings in the front inner tube is insufficient effectively to cool the combustion gas within the front inner tube. As mentioned above, 1.7 Ao to 2.5 Ao is fed through the secondary air supply channels into the zone within the front inner tube while 2.0 Ao to 2.7 Ao is supplied as diluting air into the downstream zone of the combustion chamber. As a result of such insufficient supply of air through the secondary supply openings, the production of NO.sub.x is enhanced.
It is therefore an object of the invention to provide a combustor for gas turbines in which the production of NO.sub.x gases is considerably reduced.
Another object of the invention is to provide a combustor in which the production of CO is kept at a low value.
Another object of the invention is to provide a combustor in which the combustion is stabilized.
A combustor for gas turbines according to the present invention comprises a generally cylindrical tube means defining a central axis and a combustion chamber including first, second and main combustion spaces, the portion of the tube means defining the first combustion space having a plurality of primary air supply openings, means causing a swirling stream of the primary air, and nozzle means disposed at an end of the tube means for supplying fuel to the first combustion space along the central axis, the portion of the tube means defining the second combustion space having a greater diameter than the portion defining the first combustion space and including a plurality of secondary air supply openings, and the portion of the tube means defining the main combustion space having a greater diameter than the portion defining the second combustion space and including a plurality of diluting air supply openings, wherein the tube means are so formed that the secondary air fed through the secondary air supply openings reaches the portion of the combustion chamber adjacent the central axis. The combustion gas will thus be effectively cooled even within the central portion of the combustion chamber so that the production of NO.sub.x gases is effectively reduced.
In a preferred embodiment of the invention, the primary air supplied to the first combustion space is between about 25 and about 32% of the total air supplied to the combustion chamber, the secondary air supplied to the second combustion space is between about 38 and about 50% of the total air, and the diluting air supplied to the main combustion space is below about 30% of the total air. Such distribution to the individual combustion spaces not only reduces the production of undesired NO.sub.x gases but at the same time supports a stable combustion within the combustion chamber.
The above and further objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purpose of illustration only, one preferred embodiment of the invention.